Closed loop hydraulic system with variable charge pressure

ABSTRACT

A closed loop hydraulic system having a variable charge pressure capability without parasitic power losses is disclosed. The present system includes a variable displacement hydraulic pump driven by a power source, a tilt control means for controlling a tilt angle of the variable displacement hydraulic pump, and a hydraulic motor having a pair of inlet/outlet ports connected in a closed circuit to a pair of inlet/outlet ports of the variable displacement hydraulic pump by way of a pair of main conduits. The system has a charging system including a charge pump driven by a power source, the charge pump including an inlet connected to a supply of oil, and an outlet port connected to the closed circuit for supplying pressurized oil to the variable displacement hydraulic pump. The charge system further includes an electrohydraulic proportional relief valve including means for varying an operating pressure setting thereof, which electrohydraulic proportional relief valve is connected to the charging system for controlling the pressure level of the pressurized oil supplied to the variable displacement hydraulic pump. The means for varying the operating pressure setting of the proportional relief valve are controlled by control means responsive to at least a pressure condition in the closed circuit. According to another aspect of the invention, the charging system is also connected to a hydraulic circuit of the tilt control means for supplying pressurized oil thereto.

TECHNICAL FIELD

This invention relates generally to a closed loop hydraulic systemincluding a charging system providing variable charge pressure, and moreparticularly, to a closed loop hydraulic system providing variablecharge pressure relief through an electrohydraulic proportional reliefvalve and control means therefor responsive to at least a pressurecondition in the closed circuit for providing charge flow sufficient tomeet charge requirements without parasitic power losses.

BACKGROUND ART

Parasitic power losses are a concern with closed loop hydraulic systems,including, but not limited to, hydrostatic vehicle proportion drives anddifferential steering systems. It has been found that a majorcontributor to such parasitic losses in these systems is the wastedhydraulic power of the charge flow being throttled across a chargerelief valve. Charge systems typically provide power proportional toengine power at a constant pressure for system oil makeup and controlactuation. Under some operating conditions, the charge system flow issubstantially greater than that required. For instance, at a nullcondition, that is, wherein the main pump is not providing flow, chargeflow is only needed to provide makeup for leakage and/or flushing in theclosed loop system. Thus, it has been observed that at conditions suchas the null condition the charge system flow can be significantlyreduced. However, it has previously been problematic to reduce chargeflow in a closed loop system without incurring corresponding parasiticpower losses.

Accordingly, the present invention is directed to achieving one or moreof the objectives as set forth above.

DISCLOSURE OF THE INVENTION

According to one aspect of the present invention, a closed loophydraulic system having variable charge pressure without parasitic powerlosses is disclosed. The present system includes a variable displacementhydraulic pump driven by a power source, a tilt control means forcontrolling a tilt angle of the variable displacement hydraulic pump,and a hydraulic motor having a pair of inlet/outlet ports connected in aclosed circuit to a pair of inlet/outlet ports of the variabledisplacement hydraulic pump by way of a pair of main conduits. Thesystem has a charging system including a charge pump driven by a powersource, the charge pump including an inlet connected to a supply of oil,and an outlet port connected to the closed circuit for supplyingpressurized oil to the variable displacement hydraulic pump. The chargesystem further includes an electrohydraulic proportional relief valveincluding means for varying an operating pressure setting thereof, whichelectrohydraulic proportional relief valve is connected to the chargingsystem for controlling the pressure level of the pressurized oilsupplied to the variable displacement hydraulic pump. The means forvarying the operating pressure setting of the proportional relief valveare controlled by control means responsive to at least a pressurecondition in the closed circuit and are operative to reduce the chargingsystem pressure, without incurring parasitic power losses. According toanother aspect of the invention, the charging system is also connectedto a hydraulic circuit of the tilt control means for supplyingpressurized oil thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole FIGURE is a schematic representation of a closed loop hydraulicsystem with variable charge pressure according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawing, a hydraulic system 10 is illustrated.Hydraulic system 10 is a closed loop system including a variabledisplacement hydraulic pump 12 conventionally connected in drivencommunication with a power source (not shown) by a drive shaft 14. Pump12 is of conventional construction and operation and includes aninternal tilt plate (not shown) operable for varying the displacementthereof, connected to tilt control means 16 including an actuator 18operatively connected to a variably operable pump control valve 20 vialines 22. Pump control valve 20 is operator or system controllable usinga controller 24 connected to valve 20 via wires 26 and 28. Controller 24is preferably a computerized device including a microprocessor orsimilar processor conventionally operable for controlling a tilt angleof the tilt plate of variable displacement hydraulic pump 12, and also acharging system, as will be explained.

Hydraulic system 10 includes a hydraulic motor 30 having a pair ofinlet/outlet ports 32 and 34 connected in a closed circuit to a pair ofrespective inlet/outlet ports 36 and 38 of pump 12 by a pair of mainconduits 40 and 42.

Hydraulic system 10 includes a charging system 44 including a chargepump 46 connected in driven communication with drive shaft 14. Chargepump 46 includes an inlet port 48 connected to a supply of oil 50, andan outlet port 52 connected to a supply line 54. Supply line 54 splitsat connection 56 so as to connect through a pair of check valves 58 and60 to the respective main conduits 40 and 42, check valves 58 and 60allowing flow of pressurized oil therethrough from supply line 54 to therespective conduits 40 and 42, but not in the opposite direction.Opposite direction flow is allowed past the respective check valves 58and 60 by variable pressure relief valves 62 and 64 when predeterminedpressure conditions exist in the respective conduits 40 and 42.

Supply line 54 is additionally connected to pump control valve 20 oftilt control means 16 by a line 66 for providing pressurized oil theretoas required, which line 66 connects at connection 68 to a line 70connected to a drain 72 through a relief valve 74 operable inassociation with a check valve 76 connected across conduits 40 and 42via lines 78 and 80.

Charging system 44 importantly includes an electrohydraulic proportionalrelief valve 82 having an inlet port 84 and a pilot signal port 86connected to supply line 54, and an outlet port 88 connected to drain72. Proportional relief valve 82 has an operating pressure setting whichis determined by an actuator 90 operably connected to controller 22 bywire 92. Controller 24 is operable for controlling actuator 90 forvarying the operating pressure setting of proportional relief valve 82responsive to inputs, including, but not limited to, a pressurecondition in one of the main conduits 40 or 42 corresponding to thedischarge pressure of pump 12, the operating speed of pump 12, and/oroperator inputs. A pressure sensor 94 is connected to main conduits 40and 42 at a resolver 96 for sensing a higher pressure condition in therespective conduits 40 and 42 corresponding to the disclosure pressureof pump 12, pressure sensor 94 being operable to communicate a pressuresignal to controller 24 via a wire 98. A speed sensor 100 is positionedfor sensing a rotational speed of drive shaft 14 representative to thespeed of pump 12, and generating a signal representative thereof, thesignal being communicated to controller 24 via a wire 102. Additionally,operator input is provided to controller 24 via operator input device104 connected to controller 24 via a wire 106.

In operation, controller 24 controls actuator 90 based on inputs frompressure sensor 94, speed sensor 100, operator input device 104, and anyother suitable desired parameter, including, but not limited to, anactuator torque map, or the like. The charge pressure is preferablymaintained at a variable level just above that which is adequate to meetsystem requirements, although it is also contemplated that as few as twopressure levels could be utilized, including a lower pressure settingfor null conditions of pump 12, and a higher setting when pump 12 ispumping. In either instance, since charge pressure relief is providedthrough electrohydraulic proportional relief valve 82, parasitic powerlosses due to throttling using system pressure are avoided.

INDUSTRIAL APPLICABILITY

The closed loop hydraulic system with variable charge pressure accordingto the present invention has utility for a wide variety of closed loopssystems, including, but not limited to, hydrostatic vehicle proportionaldrives and differential steer systems for reducing parasitic powerlosses in the systems.

Other aspects, objects and advantages of the present invention can beobtained from a study of the drawings, the disclosure and the appendedclaims.

What is claimed is:
 1. A hydraulic system, comprising: a variabledisplacement hydraulic pump driven by a power source, a tilt controlmeans for controlling a tilt angle of the variable displacementhydraulic pump; a hydraulic motor having a pair of inlet/outlet portsconnected in a closed circuit to a pair of inlet/outlet ports of thevariable displacement hydraulic pump by way of a pair of main conduits;a charging system including a charge pump driven by a power source, thecharge pump including an inlet port connected to a supply of oil, and anoutlet port connected to the closed circuit for supplying pressurizedoil to the variable displacement hydraulic pump; an electrohydraulicproportional relief valve including means for varying an operatingpressure setting thereof and being connected to the charging system forcontrolling the pressure level of the pressurized oil supplied to thevariable displacement hydraulic pump; and control means for controllingthe means for varying the operating pressure setting of theelectrohydraulic proportional relief valve responsive to at least apressure condition in the closed circuit.
 2. The hydraulic system, asset forth in claim 1, wherein the operating pressure setting of theelectrohydraulic proportional relief valve is variable between a highpressure setting and a low pressure setting.
 3. The hydraulic system, asset forth in claim 2, wherein the control means are operable forcontrolling the means for varying the operating pressure setting of theelectrohydraulic proportional relief valve to maintain the high pressuresetting responsive to a high pressure condition in the closed circuitand a low pressure setting responsive to a pressure conditionrepresentative of a null condition of the variable displacementhydraulic pump.
 4. The hydraulic system, as set forth in claim 1,wherein the tilt control means comprise a hydraulic circuit, and whereinthe outlet port of the charge pump is connected to said hydrauliccircuit for supplying pressurized oil to the hydraulic circuit.
 5. Thehydraulic system, as set forth in claim 1, wherein the control means areresponsive to the pressure condition in the closed circuit and a speedcharacteristic of the variable displacement hydraulic pump.
 6. Thehydraulic system, as set forth in claim 1, wherein the control meanscomprise a microprocessor operatively connected to a pressure sensor forsensing the pressure condition in the closed circuit.
 7. The hydraulicsystem, as set forth in claim 6, wherein the pressure condition is apump discharge pressure.
 8. The hydraulic system, as set forth in claim7, wherein the control means for controlling the means for varying theoperating pressure setting of the proportional relief valve are operableresponsive to the pump discharge pressure, a tilt characteristic of saidvariable displacement hydraulic pump, a speed of the pump, and anoperator input.